When using realtime-virtual-[host|guest] tuned profile, the grub parameter is not applied correct.  

It should be:

BOOT_IMAGE=(hd0,msdos1)/vmlinuz-4.18.0-151.rt13.8.el8.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap mitigations=off console=ttyS0 skew_tick=1 isolcpus=2,3 intel_pstate=disable nosoftlockup nohz=on nohz_full=2,3 rcu_nocbs=2,3

While it is actually:

BOOT_IMAGE=(hd0,msdos1)/vmlinuz-4.18.0-151.rt13.8.el8.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap mitigations=off console=ttyS0 =1 isolcpus=2,3 intel_pstate=disable nosoftlockup nohz=on nohz_full=2,3 rcu_nocbs=2,3

(NOTE: "skew_tick=1" is cut into "=1")

I feel like it's a grub thing because if we check /etc/grub2/grub.cfg we've got the tuned parameters set correct:

set tuned_params="skew_tick=1 isolcpus=2,3 intel_pstate=disable nosoftlockup nohz=on nohz_full=2,3 rcu_nocbs=2,3"

(please see below for more information)

However I'm still opening this against tuned to verify my understanding is correct.

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Versions:

[root@rt-vm ~]# rpm -qa | grep -E "tuned|kernel|grub"
kernel-4.18.0-154.el8.x86_64
kernel-rt-4.18.0-151.rt13.8.el8.x86_64
tuned-profiles-nfv-host-bin-0-0.1.20180302git1edfa966.el8.x86_64                                                                                  
kernel-modules-4.18.0-154.el8.x86_64 
grub2-common-2.02-79.el8.noarch
kernel-tools-4.18.0-154.el8.x86_64                                                                                                                
grub2-pc-2.02-79.el8.x86_64                                                                                                                       
kernel-tools-libs-4.18.0-154.el8.x86_64
grub2-tools-minimal-2.02-79.el8.x86_64
grub2-tools-2.02-79.el8.x86_64
kernel-core-4.18.0-154.el8.x86_64
kernel-rt-modules-4.18.0-151.rt13.8.el8.x86_64
tuned-profiles-nfv-guest-2.12.0-3.el8.noarch
tuned-profiles-nfv-host-2.12.0-3.el8.noarch
grub2-tools-extra-2.02-79.el8.x86_64 
grub2-pc-modules-2.02-79.el8.noarch
tuned-2.12.0-3.el8.noarch
kernel-rt-core-4.18.0-151.rt13.8.el8.x86_64
tuned-profiles-realtime-2.12.0-3.el8.noarch
tuned-profiles-nfv-2.12.0-3.el8.noarch
grubby-8.40-37.el8.x86_64

More contexts (a guest with 4 vcpus, isolating 2,3):

[root@rt-vm ~]# cat /etc/tuned/realtime-virtual-guest-variables.conf 
isolated_cores=2,3

[root@rt-vm ~]# tuned-adm active
Current active profile: realtime-virtual-guest

[root@rt-vm ~]# cat /etc/default/grub 
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto resume=/dev/mapper/rhel-swap rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap mitigations=off console=ttyS0"
GRUB_DISABLE_RECOVERY="true"
GRUB_ENABLE_BLSCFG=true
GRUB_CMDLINE_LINUX_DEFAULT="${GRUB_CMDLINE_LINUX_DEFAULT:+$GRUB_CMDLINE_LINUX_DEFAULT }\$tuned_params"
GRUB_INITRD_OVERLAY="${GRUB_INITRD_OVERLAY:+$GRUB_INITRD_OVERLAY }\$tuned_initrd"

[root@rt-vm ~]# grep tuned /boot/grub2/grub.cfg 
### BEGIN /etc/grub.d/00_tuned ###
set tuned_params="skew_tick=1 isolcpus=2,3 intel_pstate=disable nosoftlockup nohz=on nohz_full=2,3 rcu_nocbs=2,3"
set tuned_initrd=""